This series was originally entitled Contemporary Topics in Immunochemistry, and Volume 1 bearing that name was published. Upon its editorial review and while charting the development of future volumes, the editors began to sense that the word "Immunochemistry" was somewhat restrictive according to its present interpretation. Accompanying the expansion of knowledge in immuno­ biology is a demand for explanations in molecular terms. Since the intent of the series is to focus attention on research at the molecular level in any aspect of immunology, the editors and publisher felt the term "Immunochemistry" should be replaced with "Molecular Immunology." Thus, the series now bears a revised appellation, Contemporary Topics in Molecular Immunology. The editors feel this more accurately reflects the intended breath of the series. An apology is offered to writers, librarians, and other catalogers for the inconvenience this change will cause. F. P. Inman General Editor Athens, Georgia March,1973 vii Preface The earliest explorers into immunology were at once confronted by myriad molecular riddles which became increasingly complex as immunochemical techniques resolved one question only to raise scores of others. Even as our knowledge of cellular immunology was growing remarkably fast, during the past two decades exciting experiments delineated the molecular structure of immuno­ globulins. These joint advances not only shaped the Gestalt of present-day immunology, but paved the way for an incisive molecular approach to the challenges of research.